Kiyoshi Itami

Investigations of impurity seeding and radiation control for long-pulse and high-density H-mode plasmas in JT-60U

Rseduction of heat loading appropriate for the plasma facing components such as the divertor is crucial for a fusion reactor. Power handling by large radiative power loss has been studied in long pulse ELMy H-mode discharges on JT-60U (τd = 30–35 s). Case 1 is argon (Ar) seeding into standard ELMy H-mode plasmas, where large radiation loss in the confined region of the main plasma caused a change in ELM characteristics from Type-I to Type-III. Case 2 is a combination of Ar and nitrogen (Ne) gas seeding into Type-I ELMy H-mode plasmas with an internal transport barrier (ITB). For case 1, large radiation loss both from the main plasma and from the divertor was produced, and operation of Type-III ELMs was preferable to a reduction in ELM energy loss fraction (WELM/Wdia) to 0.15%. Both transient and steady-state heat loadings were reduced. Relatively good energy confinement (HH98y2 = 0.87 − 0.75) with large frad (Prad/Pabs > 0.8) and divertor plasma detachment was sustained continuously for 13.5 s. For case 2, with reduced Ar seeding to the main plasma and increased divertor radiation with Ne seeding, the ELMy H-mode plasma with an ITB had better energy confinement (HH98y2 = 0.95 − 0.8), which was sustained continuously for 12 s. The radiated power was increased primarily in the divertor ( ), which was produced both by seeded Ne ions and by carbon influx due to transient (ELM) and steady-state heat loadings in the attached divertor. Reduction in the heat loading was not enough, thus enhancement of the radiated power in the divertor will be necessary for the formation of the divertor detachment.

Highly charged tungsten spectra observed from JT-60U plasmas at Te ≈ 8 and 14 keV

We observe the vacuum ultraviolet spectra of highly charged tungsten ions in plasmas at electron temperatures of 8 keV and 14 keV. The plasmas are produced by a tokamak JT-60U. The observed emission lines in a wavelength range of 2.6–3.2 nm for the plasma at 8 keV are identified to be the 3p3/2–3d5/2 transitions of W47 +–W54 +. The observed emission lines at 2.0 nm and 2.3 nm at 14 keV are identified to be the 3p1/2–3d3/2 transitions of W55 +–W61 + and 3s1/2–3p3/2 transitions of W60 +–W63 +, respectively. The identification is obtained with the collisional-radiative calculation. Then the observed spectra are compared with the synthesized spectra with a prediction of the ionization balance. The emission lines identified here can be used for the tungsten density measurement in ITER plasmas.

Principle of an in-situ relative calibration method using a double-pass beam for Thomson scattering diagnostics

In this paper, we present the detailed principles of an in-situ relative calibration method used to determine electron temperature (Te) without using optical relative transmissivity and to obtain the optical relative transmissivity. In this method, a double-pass scattering system is used and ratio between the two signals due to forward and back scattering in each spectral channel is measured. Each signal from both the scattering must be resolved using detectors with a fast response time. Statistical simulation results indicate that the method suppresses the systematic error in Te caused by chromatic degradation in optical components because of the radiation effect. The statistical error is almost the same as that of the standard method, which uses the measured wavelength spectrum to determine the electron temperature and density from a single pass. In this new method, the scattering angle (?) and Te are considered as important parameters for evaluating its feasibility. Therefore, statistical errors for various ? were investigated. As a result, we found that the method is effective in the range of ? > 105? at Te = 10?keV. The statistical error tends to decrease for high Te measurements.

Design of collection optics and polychromators for a JT-60SA Thomson scattering system

This paper presents designs of collection optics for a JT-60SA Thomson scattering system. By using tangential (to the toroidal direction) YAG laser injection, three collection optics without strong chromatic aberration generated by the wide viewing angle and small design volume were found to measure almost all the radial space. For edge plasma measurements, the authors optimized the channel number and wavelength ranges of band-pass filters in a polychromator to reduce the relative error in T(e) by considering all spatial channels and a double-pass laser system with different geometric parameters.

ITER diagnostics: Integration and engineering aspects

ITER diagnostic equipment is integrated in six equatorial and 12 upper ports, five lower ports, and the 16 divertor cassettes directly in front of them, and at many locations in the vacuum vessel. The integration has to satisfy multiple requirements and constraints and at the same time must deliver the required diagnostic performance. Design work has been carried out in problematical and interfacing areas for most diagnostics. The main diagnostic support components, the port plugs and divertor port racks, comprise common structural elements with dedicated modules from several diagnostics. The scope of the engineering task is summarized and an overview of the integration of the diagnostic equipment is given. The engineering work to date represents the input to the port engineering tasks of the construction phase with common or repeated elements being assessed for feasibility, resolving issues, and identifying show stoppers. It has been dominated by allocation and spatial integration in the tokamak. The work that follows will be more oriented towards the acceptability for operation, suitability for function, and conformity for licensing.

Chevron beam dump for ITER edge Thomson scattering system

This paper contains the design of the beam dump for the ITER edge Thomson scattering system and mainly concerns its lifetime under the harsh thermal and electromagnetic loads as well as tight space allocation. The lifetime was estimated from the multi-pulse laser-induced damage threshold. In order to extend its lifetime, the structure of the beam dump was optimized. A number of bent sheets aligned parallel in the beam dump form a shape called a chevron which enables it to avoid the concentration of the incident laser pulse energy. The chevron beam dump is expected to withstand thermal loads due to nuclear heating, radiation from the plasma, and numerous incident laser pulses throughout the entire ITER project with a reasonable margin for the peak factor of the beam profile. Structural analysis was also carried out in case of electromagnetic loads during a disruption. Moreover, detailed issues for more accurate assessments of the beam dumps lifetime are clarified. Variation of the bi-directional reflection distribution function (BRDF) due to erosion by or contamination of neutral particles derived from the plasma is one of the most critical issues that needs to be resolved. In this paper, the BRDF was assumed, and the total amount of stray light and the absorbed laser energy profile on the beam dump were evaluated.

Progresses in development of the ITER edge Thomson scattering system

This paper includes discussions of spatial resolution and accuracy of the edge Thomson scattering system in ITER (ITER ETS). In the present design, the dominant factor for spatial resolution degradation relative to the scattering length is aberrations of the collection optics. A scattering length of approximately 4 mm is acceptable to obtain a spatial resolution of 5 mm. Statistical errors were evaluated according to measurement accuracy. Since the background light during ITER plasma discharge is much stronger than the Thomson scattering, the laser pulse duration is one of the most crucial specifications to obtain accurate measurements. The impact of fast sampling relative to current integration was also investigated. It is expected that the measurement accuracy improves when the waveform of the scattered light is sampled directly particularly for low density measurement.

Multiplexing thermography for International Thermonuclear Experimental Reactor divertor targets

The concept of multiplexing thermography is applied to the design of the divertor thermography system for International Thermonuclear Experimental Reactor (ITER). The combination of the front mirror with multiellipticity and a Czerney–Turner spectrometer with a 0.2 mm pitched multichannel detector enables a spatial resolution of 3 mm and a time resolution of 20 μs above a target temperature of 300 °C to be achieved. This should be sufficient to measure ELM heat fluxes to the targets in ITER. To satisfy the measurement requirement, it is very important to keep an accurate alignment around the optical axis against movement of the vessel during the plasma discharges. Several key engineering problems, such as the survivability of components against mirror coating by redeposited divertor material, remain to be solved. Potential solutions have been identified.

Development of laser beam injection system for the Edge Thomson Scattering (ETS) in ITER

This paper focuses on the design and development of the laser injection system for the ITER Edge Thomson Scattering system (ETS). The ITER ETS achieves a temporal resolution of 100 Hz by firing two 50 Hz laser beams alternatively. The use of dual lasers enables us to perform the Thomson scattering measurements at a temporal resolution of 50 Hz in case that one of the laser systems stops functioning. A new type of beam combiner was developed to obtain a single beam that is collinear and fixed linearly polarized from two laser beams using a motor-driven rotating half-wave plate. The rotating half-wave plate method does not induce misalignment even if the rotating mechanism malfunctions. The combined beam is relayed from the diagnostic hall to the plasma using mirror optics and is absorbed at the beam dump integrated on the inner blanket. The beam alignment system was designed to direct the laser beam onto the center of the beam dump head. The beam position at the beam dump is monitored by four alignment laser beams which propagate parallel to the diagnostic Nd:YAG laser beam and imaging systems installed outside the diagnostic port.

Construction of a fuel retention model for full carbon devices

Hydrogen and deuterium retention in the plasma-facing carbon tiles used as divertor plates, baffle plates (BPs) and first wall (FW) tiles of JT-60U, which is a full carbon device and operated at 573 K, was measured by means of thermal desorption spectroscopy and a model was constructed for fuel retention including the whole area of JT-60U. The total retention was split into three components: (i) re-deposited layers on the divertor plates, (ii) eroded tiles (most of the area of the outer divertor plate, the BP and the FW) and (iii) the bulk of all tiles. For long discharge time, retention in the re-deposited layers dominated the total retention because of the linear increment of the thickness of the re-deposited layers with increasing discharge time. However, the fuel retention rate for all components is smaller than other estimates in the literature owing to the higher temperature operation of JT-60U.